topic 11 electrolysisnew
TRANSCRIPT
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ELECTROLYSIS
Mohamad nasir bin othman
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INTRODUCTION
In contrast to spontaneousredox reaction, which result inthe conversion of chemicalenergy into electrical energy,electrolysis is the process in
which electrical energy is usedto cause a non-spontaneouschemical reaction to occur.
An electrolytic cell is anapparatus for carrying outelectrolysis. The same
principle underlie electrolysisand the processes that takeplace in galvanic cells.
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Electrolyte cell
The electrolyte cell contains a pair of
electrode connected to the battery.
The battery serves as an electron pump,
driving electrons to the cathode, where
reduction occurs, and withdrawing electrons
from the anode, where oxidation occurs.
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The reactions at the electrodes are,
Anode (oxidation): 2Cl-(l) Cl2(g) + 2e
Cathode (reduction) 2Na
+
(l) + 2e 2Na(l)Overall: 2Na
+(l) + 2Cl
_(l) 2Na(l) + Cl2(g)
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Electrolysis of Water
19.8
Anode: 2H2O (l) O2(g)+ 4H+(aq)+ 4e-
Cathode: 4H+(aq) + 4e 2H2(g)
2H2O (l) 2H2(g) + O2(g)
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Electrolysis
Electrolysis of moltencompounds
Metals are formed at the cathode
Non-metals are formed at theanode
Electrolysis of aqueoussolution
Selective discharge of ions determined by:
Position of ions in the electrochemical series
Concentration of ions
Types of electrodes
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Electrolysis of molten compounds
Example 1 : PbBr2
Anode: ?
Katode: ?
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a. Position of ions in the electrochemical
series
The lower the position
of the ion in the
electrochemical series,
the easier the ion willbe discharged.
Example: Electrolysis of
aqueous sodium sulphate
(Na2SO4).
Na2SO4 2Na+ + SO4
2-
H2O = H+ + OH
-
Anode : ?
Cathode: ?
Cation Anion
K+
Na+
Mg2+
Al3+
Zn
2+
Fe3+
Sn2+
Pb2+
H+
Cu
2+
Hg+
Ag+
F-
SO42-
NO3-
Cl-
Br
-
I-
OH-
Tendency
to
discharge
increases
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b. Effect of concentration of ions in the
solution, CuCl2(aq)
CuCl2 Cu2+
+ 2Cl-
H2O H+
+ OH-
Anode: ?
Cathode: ?
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c. Effect of types of electrode used
Electrolysis of CuSO4 solution
CuSO4 Cu2+
+ SO42-
H2O H+
+ OH-
(a) If carbon is used as the electrodes, OH-
ions aredischarged at the anode because of the position of OH-ion in the electrochemical series.
(b) if Cu is used as the anode,both SO42-
and OH- ions
are not discharged. Cu acts as an active electrode here
because it takes part in the chemical reaction duringelectrolysis.
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Quantitative aspects of electrolysis
The quatitative treatmentof electrolysis wasdeveloped primarily byFaraday.
He observed that the massof product formed (orreactant consumed) at anelectrode is proportional toboth the amount of
electricity transferred at theelectrode and the molarmass of the substanceinquestion.
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The amount of electricity transferred is measuredin the unit of coulomb (C).
One coulomb is the quantity of electric charge
transferred by a current of one ampere (A) in onesecond (c). Thus,
Q = It
Q : quantity of electric charge (C)
I :current (A)
t : time (s)
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Faraday constant (F)
Faraday constant (F) is the charge on one mole
of electrons, that is
1 F = 96500 C
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Faradays First Law
Faradays First Law states that the mass of a
substance produced at an electrode during
electrolysis is proportional to the quantity of
electricity (in coulomb) passed.
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Example 1
An aqueous solution of copper(II)sulphate is
electrolysed using a current of 0.150A for 5
hours. Calculate the mass of copper deposited
at an electrode at the cathode.
Q = it = 0.150 x 5 x 3600 = 2700 C
Number of moles of electrons = 2700/96500 = 0.028 mol
Number of moles of copper deposited= x 0.028 = 0.014 mol
Mass of copper deposited = 0.014 x 63.5 g = 0.889 g
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Faradays Second Law
If the same quantity ofelectricity is passedthrough differentelectrolytes, the mass ofthe substance liberated atthe electrode is inverselyproportional to thecharges on the ions.
Thus, 1F of electricity will
discharge 1 mole of Ag+
ions, but mole of Cu2+ions, 1/3 mole of Al3+ ionsand mole of oxygen gas.
We can determine the mass of a substance
deposited at an electrode by weighing the
electrode before and after the electric current
Is supplied. The apparatus used for this type of
experiment is called coulometer.
The apparatus on the right is called a voltameter.A voltameter is
an electrolyte cell which is set up for collecting and measuringthe volume of any gas liberated during the electrolysis.
Figure 1
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Example 2
With reference to Figure 1,calculate
(a) the mass of copperdeposited
(b) the volume (in dm3)ofhydrogen liberated
(c) the volume (in cm3 )of oxygen
collected at room temperatureand pressure, if 0.32 gof silver isdeposited in the silvercoulometer.
Answer: (a) 0.094g (b) 0.0355dm3(c) 17.8 cm3
Figure 1
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(a) Number of moles of silver deposited = 0.32/108 = 2.96 x 10-3 mol
1F = 1mol e = 1 mol Ag = mol Cu2+Number of moles of Cu deposited = x 2.96 x 10-3
= 1.48 x 10-3 mol
Mass of Cu deposited = 1.48 x 10-3 mol x 63.5 g/mol
= 0.094 g
(b) 1 F = Ag+= H2Number of moles of H2liberated = x 2.96 x 10
-3
= 1.48 x 10-3 mol
Volume of H2liberated = 1.48 x 10-3
mol x 24
= 0.0355 dm3
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(c) 2O2- O2 + 4e
Number of moles of O2liberated = x 2.96 x 10-3
= 7.40 x 10-4 mol
Volume of H2liberated = 7.40 x 10-4 mol x 24 x 1000
= 17.8 cm3
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Miskonsepsi Pelajar 1
Anion sudah mencapai keadaan oktet; sudah
stabil, mengapa masih berlaku tindakbalas
pada anod sel elektrolisis?
Miskonsepsi: Anion stabil tetapi bersifat
nukleofilik (kaya elektron) dan juga reaktif; ia
gemar untuk mendermakan elektronnya bagi
membentuk kestabilan baru melalui ikatankovalen.
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Miskonsepsi Pelajar 2
Kenapa dalam sel elektrolisis kation pergi ke
katod; tidak ke anod?
Jawapan: Kation bercas positif dan katod
bercas negatif. Cas +ve danve akan saling
tarik menarik.